Excerpts from Books and Wikipedia
"The chief problem of Æthelred's reign [978-1013 and 1014-1016]was conflict with the Danes. After several decades of relative peace, Danish raids on English territory began again in earnest in the 980s. Following the Battle of Maldon in 991, Æthelred paid tribute, or Danegeld, to the Danish king. In 1002, Æthelred ordered what became known as the St. Brice's Day massacre of Danish settlers. In 1013, King Sweyn Forkbeard of Denmark invaded England, as a result of which Æthelred fled to Normandy in 1013 and was replaced by Sweyn. However, he returned as king for two years after Sweyn's death in 1014. Æthelred's 37-year reign was the longest of any Anglo-Saxon king of England, and was only surpassed in the 13th century, by Henry III. Æthelred was briefly succeeded by his son, Edmund Ironside, but he died after a few months and was replaced by Sweyn's son, Cnut. Another of his sons, Edward the Confessor, became king in 1042." [Wikipedia] Ethelred married Emma in 1002 to make an alliance with the Normans, but he could still not "muster a force large enough to turn the Danes away." [Bauer: Medieval World, p. 526-8]